std::insert_iterator and std::inserter need to be adjusted for C++20, so
that they use ranges::iterator_t. That alias template requires
ranges::begin to be defined. Rather than moving the whole of
ranges::begin (and related details like ranges::enable_borrowed_range)
into <iterator>, this defines a new, simpler version of ranges::begin
that is sufficient for ranges::iterator_t to be defined. This works
because ranges::iterator_t uses an lvalue reference type, so the logic
in ranges::begin for non-lvalue ranges (i.e. borrowed ranges) isn't
needed.

This also adds the missing constexpr specifiers to the other insert
iterators.

	* include/bits/iterator_concepts.h (__detail::__decay_copy)
	(__detail::__member_begin, __detail::__adl_begin): Move here from
	<bits/range_access.h>.
	(__detail::__ranges_begin, __detail::__range_iter_t): Define.
	* bits/range_access.h (__cust_access::__decay_copy)
	(__cust_access::__member_begin, __cust_access::__adl_begin): Move to
	<bits/iterator_concepts.h>.
	(ranges::iterator_t): Use __detail::__range_iter_t.
	* include/bits/stl_iterator.h (back_insert_iterator): Simplify
	conditional compilation. Add _GLIBCXX20_CONSTEXPR to all members.
	(front_insert_iterator): Likewise.
	(insert_iterator): Implement changes from P0896R4 for C++20.
	* testsuite/24_iterators/back_insert_iterator/constexpr.cc: New test.
	* testsuite/24_iterators/front_insert_iterator/constexpr.cc: New test.
	* testsuite/24_iterators/headers/iterator/synopsis_c++17.cc: Adjust
	for inclusion in synopsis_c++20.cc which expects different signatures
	for some function templates.
	* testsuite/24_iterators/insert_iterator/constexpr.cc: New test.